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Probing anisotropic intermolecular forces in nematic liquid crystals using NMR and computer simulations Syvitski, Raymond Thomas

Abstract

Molecules of similar size and shape, but with different electrostatic properties are used to investigate the effects of molecular dipoles, quadrupoles and polarizabilities on the orientational ordering of several solutes co-dissolved in nematic liquid crystals. Permanent dipoles have a negligible influence on solute orientational order and effects from molecular polarizability interactions could not be separated from short-range interactions. Order parameters predicted from strong, short-range repulsive forces coupled with interactions between the solute quadrupole and the average electric field gradient felt by the solute (EFG) are consistent with experimental values. For liquid crystals utilized in this study, the calculated values of the (EFGys are the same sign and of similar magnitude to the (_FG)'s determined previously from experiments on D₂ and HD. However, in contradiction to these experimental results, the (EFG)'s determined from computer simulations of hard particles with embedded point quadrupoles is found to be very dependent on the properties of the particle. For a particular nematic liquid crystal (55 wt% ZLI 1132 in EBBA), the contribution to solute ordering from long-range electrostatic interactions is found to be negligible. This conclusion is supported by computer simulation studies of hard particles; models for short-range interactions which best fit the NMR experimental solute order parameters also best fit the simulation results. Experimentally determined second rank orientational order parameters and structural parameters of solutes are calculated from vibrationally and non-vibrationally corrected nuclear dipolar coupling constants; accurate dipolar couplings are obtained from analysis of the high-resolution nuclear magnetic resonance (NMR) spectra. For the more complicated molecules spectral parameters are first estimated from analysis of multiple quantum NMR spectra. In some cases, a modified version of a least-squares routine which independently adjusts chemical shifts, order parameters, structural parameters and/or dipolar couplings is used.

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